This invention relates to synthetic lubricating oil compositions which exhibit desirable viscosity at high temperatures and good fluidity at low temperatures and, moreover, excellent wear resistance. The invention is more particularly concerned with wear-resistant, synthetic lubricating oil compositions based on synthetic oils suited for the lubrication of mechanical superchargers of automobiles.
For greater power output and less fuel consumption by automotive engines, turbochargers and more efficient superchargers have in recent years been rapidly developed. More recently, the development of superchargers quicker to respond than heretofore is actively under way which will overcome the time lag of the turbocharger that uses exhaust gas to drive turbines which, in turn, drive an air compressor (centrifugal type air pump) to supercharge the engine.
Unlike the turbocharger, the supercharger does not exploit the exhaust gas. Its mechanism is such that, as illustrated in FIG. 5, the rotation of the engine crankshaft is transmitted through a toothed belt (not shown) to a pulley 2, and the rotation of the pulley 2 is transmitted through a gear train 4 to an air compressor (positive displacement air pump) 6, whereby air is compressed before being charged into the engine. Usually, the supercharger employs a two-lobe Roots compressor for this purpose and hence is sometimes known as Roots supercharger.
In the manner described the supercharger is directly coupled to the engine, and it offers the advantages of good engine response to the accelerator depression and improved engine efficiency and fuel economy when running in the low-speed range.
The turbocharger and supercharger, operating with different mechanisms as described above, require lubricating oils dissimilar in quality. The former needs heat-resistant oil because it handles hot exhaust gases. The oil for the latter must be resistant not only to heat but also to abrasion under the conditions of high speed running, since the gear-train drive 4 and bearings 8 are subjected to high temperatures (e.g., 150.degree. to 200.degree. C.) and high-speed rotation (e.g., 9,000 rpm).
On the other hand, automobiles should be easily driven by ordinary persons as well as by the skilled. Their parts, gears, and mechanisms must work jointly to enable the vehicles to start and run smoothly in varied driving environments, hot or cold.
Thus, the lubricating oil for the supercharger must meet the following essential requirements:
(1) High stability at elevated temperatures (about 100.degree. C.) and high-speed rotations (e.g., 9,000 rpm) PA1 (2) Good fluidity at low temperatures (e.g., -40.degree. C.) PA1 (3) Good wear resistance PA1 (4) Freedom from maintenance PA1 (5) Minimum oil volume PA1 (A) a diester of an aliphatic dibasic acid having 4 to 14 carbon atoms in the molecule and an alcohol having 4 to 14 carbon atoms or a mixture of several such diesters and having a viscosity at 100.degree. C. of 2 to 7 mm.sup.2 /s, and PA1 (B) a polyoxyalkylene glycol whose alkylene group contains 2 to 5 carbon atoms or a mixture of several such glycols and having a viscosity at 100.degree. C. of at least 20 mm.sup.2 /s,
Lubricating oils introduced so far for use over relatively wide temperature ranges have included, for example, a hydraulic oil composition based on an ester mixture described in the specification of Japanese Patent Application Public Disclosure No. 127484/1977. However, the base oil solidifies at low temperatures, e.g., between 0.degree. and -20.degree. C., and lacks the low-temperature fluidity required of the base oil for the lubricating oil of the present invention. Mineral lubricating oils with high degrees of low-temperature fluidity are already in use, e.g., as automatic transmission fluids. They are not as viscous as high temperatures, however, and are not capable of service under high-speed operating conditions that demand accordingly high wear resistance.
For use in gas turbines of aircraft, there have been proposed, and have come into use, synthetic lubricating oil compositions based on diesters and thickened with polyglycol ethers [U.S. Pat. No. 2,944,973; Journal of the Institute of Petroleum, 47, 446, P. 42 (Feb., 1961); ibid., 50, 491, p. 284 (Nov., 1964)]. However, these synthetic oils have viscosities of only about 7.5 mm.sup.2 /s at 100.degree. C. and are not effective for the high-performance superchargers of internal combustion engines recently developed or under development.
Out of the lubricating oils on the market, apparently utilizable for automotive superchargers are "ATF-DII" (automatic transmission fluid of the "Dexron II" grade) and gear oil with a service viscosity range of 75W-90. The former exhibits favorable low-temperature fluidity but inadequate viscosity at high temperatures. The latter is adequately viscous at high temperatures but has a drawback of excessive viscosity and poor fluidity at low temperatures. (Refer to FIG. 1.) Moreover, the latter lubricating oil, 75W-90, requires the addition of a viscosity index improver to enhance its high-temperature viscosity at the expense of wear resistance.
None of the commercially available lubricating oils have been found capable of combining good viscosity at high temperatures (e.g., 100.degree. C.) with good fluidity at low temperatures (e.g., -40.degree. C.) without the incorporation of a viscosity index improver.
Conventional lubricating oils, usually based on mineral oils, deteriorate so rapidly that frequent oil replacement is necessary. In an effort to retard the mineral oil deterioration it has been customary to circulate a large volume of the oil composition.
As will be understood from the foregoing, the lubricating oil compositions taught by the above-mentioned patent were defective in that the base oil solidifies at low temperatures, e.g., at 0.degree. to -20.degree. C., or fails to remain sufficiently viscous at elevated temperatures for service under the conditions of high-speed rotation. Also, among the automatic transmission fluids and gear oils in use, there is not a single product excellent in all three essential properties; low-temperature fluidity, high-temperature viscosity, and wear resistance.
It is therefore a principal object of the present invention to provide a wear-resistant synthetic lubricating oil having a lower rate of viscosity variation than conventional oils with a wide range of temperature changes.
To be more exact, the invention has for an object the provision of a wear-resistant, synthetic lubricating oil composition which possesses good high-temperature stability and low-temperature fluidity, for example, combining the high-temperature viscosity of the 75W-90 gear oil with the low-temperature fluidity of "ATF-DII", and yet requires no addition of a viscosity index improver.
Another object of the invention is to provide a wear-resistant, synthetic lubricating oil composition, usable particularly for automotive superchargers, which is resistant to heat and wear under high-speed running conditions and maintenance-free or capable of service for long periods without the need of replacement under conditions of high-speed rotation (e.g., 9,000 rpm) at oil temperatures between 150.degree. and 200.degree. C.